The surface of fine, inorganic solid particles is frequently coated in order to modify specific properties, such as abrasion resistance, surface charge, dispersing properties, acid or light resistance. For example, U.S. Pat. No. 2,885,366 describes the application of a dense silicon dioxide coating to substrate particles, such as nickel or iron powder, glass fibers or titanium dioxide. Color and white pigments are regularly coated with various oxides and hydroxides (e.g. EP 0 130 272 A1 and Re. 27,818).
Surface treatment, particularly of TiO2 pigments, is customarily performed in the aqueous phase, where metal oxides, hydroxides, phosphates or similar compounds are deposited on the particle surface. The method is customarily operated as a batch process and starts with an aqueous pigment particle suspension which first of all is disagglomerated, customarily in a media mill. Subsequently, metal salts are added in dissolved form as so-called precursors, and the pH value of the suspension is set by means of alkaline or acidic substances in such a way that the precursors are precipitated as oxides, hydroxides, etc.
In the classical method, there is the risk of particle reagglomeration in the suspension, meaning that the deposited coating substances do not enclose the individual particle, but often an agglomerate. The agglomerates are disintegrated again during final dry milling, as a result of which not all particles in the end product are provided with a continuous skin, but instead also display uncoated surface areas. Moreover, part of the coating substances is not fixed on the particle surface, but forms floccules alongside the particles. These floccules can no longer be removed from the suspension and have a negative impact on the optical properties of the pigments, such as the tinting strength (TS).
U.S. Pat. No. 5,993,533 discloses a method in which titanium dioxide pigment is coated with SiO2 and Al2O3 in an inline mixer. Treatment is performed in two consecutive maturation stages at 80 to 100° C. and pH values of >9.5 and <8.5.
GB 1 340 045 describes a batch method for coating the surface of titanium dioxide pigment, where a suspension of the pigment is subjected to intensive agitation in a mixing vessel for up to two hours, during which time the coating substances are added and applied.
US 2008/0141905 A1 describes a further method, in which inorganic particles are surface-coated during continuous feeding through an agitator mill. Particularly smooth and homogeneous surface-coating of the individual particles is achieved in this case.